cross-build: fix missing <sys/md4.h> the proper way

Add it to tools/build/Makefile with a short note of where it's needed,
rather than hamfistedly copying it into the tools/build hierarchy.

Reported by: jrtc27
Reviewed by: jrtc27
Fixes: aad507854efd13c43 ("Fix the cross-build after recent commits")
Differential Revision: https://reviews.freebsd.org/D46854